According to POSIX.1-2001:

"The snprintf() function shall be equivalent to sprintf(), with the addition of the n argument which states the size of the buffer referred to by s. If n is zero, nothing shall be written and s may be a null pointer. Otherwise, output bytes beyond the n-1st shall be discarded instead of being written to the array, and a null byte is written at the end of the bytes actually written into the array."